Conventional memory cells are configured to read and write data by applying a voltage to a memory material. A voltage is applied to the memory material through electrodes coupled to each end of the memory cell. The memory material is set to a particular resistance state according to an amount of current applied by the electrodes. The resistance state of the memory material may be used to distinguish a logic value of the memory cell.
In addition to the memory material, a conventional memory cell may also include an isolation element (e.g., a switch, a select device, etc.) configured to be reversibly electrically switched from a resistive state to a conductive state. Within a conventional memory device, a plurality of memory cells is positioned between a plurality of access lines (e.g., word lines) and a plurality of digit lines (e.g., bit lines). A single cell is selected for reading and writing by applying a voltage between the access line and the digit line associated with a particular memory cell. Including the isolation element impairs or, ideally, prevents, residual voltages from affecting the physical state (e.g., the resistance) of non-selected memory cells.
Threshold switching materials are currently considered favorable isolation elements, such as in, for example, cross-point architecture memory cells. At a threshold voltage, the threshold switching material changes to an electrically conductive state, allowing current to flow through the threshold switching material. Below the threshold voltage, the threshold switching material is in a resistive state, limiting leakage current flow through the threshold switching material.
The threshold switching material may be formed between a pair of electrodes of the memory cell, through which current flows to and from the threshold switching material. Conventional threshold switching materials include materials that undesirably react with the materials of the electrodes surrounding the threshold switching materials. The threshold switching materials often react with the materials that form the electrodes. In addition, the threshold switching material may diffuse into the electrode and materials from the electrode may diffuse into the threshold switching material, forming a discontinuous interface between the electrodes and the threshold switching material. For example, this diffusion of materials occurs in conventional memory cells at an interface between a metal electrode and an amorphous silicon threshold switching material, or at an interface between a carbon electrode and a chalcogenide threshold switching material.
Disadvantageously, however, these reactions between the threshold switching material and the surrounding electrodes cause electrical defects at the interface between the threshold switching material and the electrodes, reducing the electrical quality of the switch and the associated memory cell. For example, the poor interface may cause a phenomenon known as Fermi level pinning, which often increases the threshold voltage of each memory cell, increases the threshold voltage variability across individual memory cells within a memory array, increases the leakage current through the threshold switching material at sub-threshold voltages, and reduces the useful lifetime of the switch.